Muscle spindles in small hand muscles of the monkey (Macaca fascicularis) will be examined using histologic and ultrastructural techniques. The spindles will be serially thin and thick cross-sectioned in a plastic embedding medium. The sections will be stained to emphasize the somatic motor axons and their intrafusal nerve terminals. A systematic survey and quantitative analysis of motor innervation of entire spindles and individual intrafusal muscle fibers will be undertaken. The branching pattern of motor axons within the spindles will be reconstructed in detail. Quantitatively precise data will be obtained on the number, length and location of diverse motor nerve terminals along the individual intrafusal fibers. Ultrastructural characteristics of intrafusal motor endings will be described. A large number of monkey spindles will be surveyed to establish both common and uncommon patterns of spindle motor organization at the organ and intrafusal fiber levels. The work will aid our understanding of how motor control of muscle spindles in the macaque is effected. The study will represent the first detailed analysis of the motor innervation of muscle spindles in primates. As such, it will facilitate understanding of peripheral motor control in humans, both in health and disease.